1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory and a method of operating the memory. More particularly, the invention relates to a non-volatile semiconductor memory which can be highly integrated and has cells of a contactless structure, and a method of operating the memory.
2. Description of Related Art
In a rewritable non-volatile NOR-type memory array of prior art, as shown in FIG. 14, each cell transistor 15 is formed in an active region defined by a field oxide film 16. The transistor 15 is composed of a source 11/drain 12, a floating gate 17 formed on the active region between the source 11/drain 12, and a control gate 10 formed on the floating gate 17 and connected to a word line perpendicular to a channel direction between the source 11/drain 12. In this memory cell array, the drain 12 is connected to a bit line 14 via a contact 13.
Thus the contact 13 formed on the drain 12 occupies a large part of a cell area and prevents the reduction of the cell area.
There is proposed a contactless memory array which is not provided with a contact for connecting the source/drain with the bit line, as shown in FIG. 15. In this memory cell array, a bit line 20 is formed as an impurity diffusion region which also functions as the source/drain.
However, in this memory array, since a word line 17 is arranged in the same direction as the channel direction of a cell transistor 18, a cell current at reading is affected by an adjacent cell on the same word line 17. Accordingly, even if a specific one of a plurality of cells on the same word line is written and the threshold of the specific cell is accurately controlled, the following problems arise:
(1) If the threshold of an adjacent cell on a source side changes from a low value to a high value, an apparent source resistance increases and the threshold of the specific cell shifts from its original value to a higher value.
(2) If the threshold of an adjacent cell on a source side changes from a high value to a low value, an apparent source resistance decreases and the threshold of the specific cell shifts from its original value to a lower value.